Developments in steel production in electric arc furnaces have been marked since the last oil crisis by measures for saving electrical energy and shortening the melting-down time. Thus for example, in Japan in the period from 1973 to 1983, the average duration of the tapping cycle has been reduced, for 146 electric furnaces, from 2 hours 46 minutes to 1 hour 51 minutes. The power used was reduced in the same period from 543 to 439 kWh per ton. Electrode consumption fell from 5.1 to 3.1 kg/t.
To speed up the melting down of scrap, oxygen is blown in through consumable pipes or lances on to the hot scrap in amounts of up to 25 Nm.sup.3 /t. The iron oxide formed is partly reduced after the melting down of the scrap by the addition of fairly small amounts of lump coke or by blowing in coal, also through consumable lances. However, there has still only been a small gain in heat with this conversion of the relatively small amounts of carbon into CO.
The melting down of scrap is also speeded up by the use of various types of burners, in particular oil-oxygen burners. These burners are usually arranged immediately above the melt in the upper part of the side walls of the furnace.
In the past there has been no lack of efforts to reduce the economically important consumption of graphite electrodes by means of electrodes without through-flow or of hollow electrodes fed with gas or gas-solid suspensions, and/or to produce other effects such as lowering the hydrogen content in the steel or the addition of solids generally. U.S. Pat. No. 2,909,422 and German Offenlegungsschrift 29 00 864 describe such processes. The reason why this technique of blowing in through hollow electrodes has not become established lies, inter alia, in the fact that the solids that are blown in are not completely distributed in the bath owing to the lack of movement of the bath. No economically worthwhile results could be obtained by blowing in fuels.
In other present-day variants of the process, the off-gases are collected and used to heat the scrap, outside the electric arc furnace, in special preheating chambers. The hot scrap is then charged into the electric arc furnace. With these known improvements the tapping cycle of the electric arc furnace can, in individual cases, be shortened to about 80 minutes.